How to get the duration between days in PHP

Following is the funcation can calculate the duration, actually number of days between the given parameters (from,to).

The argument must be subclass of DateTime.

  1: function differenceInDays ($firstDate, $secondDate){

  2:     $firstDateTimeStamp = $firstDate->format("U");

  3:     $secondDateTimeStamp = $secondDate->format("U");

  4:     $rv = round ((($firstDateTimeStamp - $secondDateTimeStamp))/86400);

  5:     return $rv;

  6: }